1. Field of the Invention
The present invention relates to a composition having sensitivity to light or radiation, wherein the term "light" or "radiation" hereinafter represents visible light, ultraviolet rays, far ultraviolet rays, X-rays, an electron beam, or an ion beam, a process for forming a pattern, a method for preparing a photomask, and a method for manufacturing a semiconductor device by using the same.
2. Description of the Prior Art
There has arisen a demand for the formation of a predetermined fine resist pattern in the lithography process because of the tendency toward miniatuarization of semiconductor devices in recent years. In the lithography process, a resist film applied to a substrate is exposed to light through a mask possessing light non-transmissive portions and light transmissive portions. In this process, light is emitted from an exposure and reduction projection apparatus, through a pattern on a mask which is reduced by means of a lens, and then transfers such a pattern to the resist film.
FIG. 16(a) represents the chemical formula of the main component of a conventional resist.
The above resist material is mainly composed of a polymer material soluble in an alkali aqueous solution such as novolak resin or phenol resin. In addition to this polymer, a sensitizing agent such as a naphthoquinonediazido compound is mixed therein to provide a resist material having sensitivity to light or radiation. For this resist material, an alkali aqueous solution, mainly, several percent ammonium hydroxide aqueous solution, is used as a developer. When irradiated with light or radiation, the naphtoquinone compound as a sensitizing agent produces a carboxylic acid which dissolves in an alkali aqueous solution as a developer to form a resist pattern. Consequently, the resist material which contains a naphtoquinone compound may serve as a positive type of resist material.
However, since the above resist material having sensitivity to light or radiation has low sensitivity as a resist, it needs to be irradiated with a relatively large amount of light or radiation so as to become soluble in a developer. This entails a deficiency in that the formation of the desired resist pattern requires the resist material to be irradiated with light or radiation for a relatively long period of time. Since the resist material has high absorbance to ultraviolet rays, it has another deficiency of being unsuitable for lithography with the use of light with a shorter wavelength such as far ultraviolet rays.
In order to overcome the above deficiencies, a resist which is referred to as a chemically sensitizable resist has been developed. The material for this resist comprises a sensitizing compound which produces an acid when irradiated with light or radiation (an acid producing agent), and a compound which undergoes a chemical reaction under the presence of an acid. Therefore, the dissolving rate of the resist in a developer is not directly changed by the presence of an acid resulting from an acid producing agent irradiated with light or radiation as a sensitizing agent, but changed by the reaction of other resist components catalyzed by the resulting acid. In such a resist, the change in a sensitizing agent induced when irradiated with light or radiation is amplified by catalysis of the acid, thereby making a resist having high sensitivity.
Such a chemically sensitizable resist is described in U.S. Pat. No. 4,491,628. The resist comprises a tert-butyl ester polymer as a main component and onium salt as an acid producing agent. FIG. 16(b) shows an example of the chemical formula of the components of the resist which uses tert-butoxycarbonyloxystyrene as tert-butyl ester, and triphenylsulfoniumfluoroantimonate as an acid producing agent. In this resist, when irradiated with light or radiation, the onium salt produces an acid which catalyzes hydrolysis of tert-butyl ester, thereby changing the property of the irradiated portions of a resist film from lipophilic property to hydrophilic property. Consequently, the irradiated portions of the resist film dissolves in a developer to form a resist pattern.
However, because of the trend for a fine structure in semiconductor devices in recent years, a fine resist pattern cannot be formed on a substrate with a large difference in level even by the use of the above chemically sensitizable resist.
For overcoming such a deficiency, multilayer resist methods have been developed. In these methods, a thick organic film is formed on a substrate to level the uneven surface of the substrate, on which a thin resist film is provided, after which the resist film is irradiated with light or radiation to form a resist pattern. Thereafter, by the use of the resist pattern as a mask, the lower thick organic film is etched by means of dry etching, thereby transferring the resist pattern to the organic film. According to these multilayer resist methods, the resolving power of the resist can be maintained, and prevented from decreasing due to the variety of the depth of focus, thereby forming a fine resist pattern on the substrate with a large difference in level. Examples of multilayer resist methods include two methods as follows:
One is a three-layer resist method, wherein in order to transfer a resist pattern to a lower thick organic film, an intermediate layer such as a silicon dioxide film having a resistance to dry etching with oxygen is interposed between the lower thick organic film and the upper resist film.
Another is a two-layer resist method, wherein a material having a resistance to dry etching with oxygen serves as an upper resist film.
The later two-layer resist method is more promising because of its easier process as compared to the former three-layer resist method. Most of the resist materials used in the two-layer method contains silicon. Since silicon is stable to oxygen plasma for use in dry etching, the silicon-containing resist film is scarcely removed by etching. Consequently, only the lower thick organic film can be etched by oxygen plasma through a silicon-containing resist pattern as a mask, thereby transferring a resist pattern with high resolution to the organic film.
The silicon-containing resists used in the two-layer resist method may be classified into three kinds as follows: (1) a polymerizable one-component resist, (2) a two-component resist containing a naphthoquinone compound as a sensitizing agent, and (3) a chemically sensitizable resist.
(1) An example of a polymerizable one-component resist containing silicon is described in Jpn. J. Appl. Phys. (pages L659-660, vol. 22, 1983, by M. Morita, et.al.), which is an SNR (Silicon-based Negative Resist) having polysiloxane bonds as shown in FIG. 17(a). This resist has sensitivity to an electron beam and far ultraviolet rays as a negative type of resist. The sensitivity to an electron beam is 4.5 .mu.C/cm.sup.2. An organic solvent such as a mixed solvent containing diisobutylketone and cyclohexanone is used as a developer. In addition to this resist, many polymerizable one-component resists containing silicon have been developed. For example, it has been reported in SPIE (pages 70-73, vol. 539, 1985, by R. G. Brault, R. L. Kubena and R. A. Metzger) that polysilsesquioxane having a trimethylsilyl group as an end group thereof as shown in FIG. 17(b) may serve as the one-component resist. The sensitivity to an electron beam of this resist is 40 .mu.C/cm.sup.2. An organic solvent such as ethyleneglycolmonoethylether is used as a developer.
(2) An example of a two-component resist comprising a naphthoquinone compound as a sensitizing agent and a silicon-containing resin which is soluble in an alkali solution as a base polymer is described in J. Electrochem. Soc. (pages 909-913, vol. 132, 1985, by Y. Saotomo, et.al.), which comprises, for example, a novolak resin that contains a trimethylsilylmethyl group shown in FIG. 17(c) as a base polymer and a naphthoquinone compound also shown in FIG. 17(c) as a sensitizing agent. Since the base polymer has a hydroxyl group, the resist is soluble in an alkali aqueous solution. Therefore, an alkali aqueous solution such as a solution of tetramethylammonium hydroxide (TMAH) is used as a developer.
FIG. 17(d) shows a resist described in SPIE (pages 291-294, vol. 920, 1988, by S. Imamura et al.), which comprises acetylated polyphenylsilsesquioxane as a base polymer, which is also soluble in an alkali aqueous solution. Consequently, the resist may serve as a positive type of resist which may be developed by the use of an alkali aqueous solution by adding a naphthoquinone compound as a sensitizing agent thereto. The sensitivity to ultraviolet rays of the resist is 100 mJ/cm.sup.2 for a g-line, and 40 mJ/cm.sup.2 for an i-line.
In addition to these resists, many resists have been developed which comprises a naphthoquinone compound as a sensitizing agent, and a silicon-containing resin which is soluble in an alkali solution as a base polymer. These resists are characterized in that they can be developed by an alkali solution and have a sensitivity of almost 100 mJ/cm.sup.2 because of the presence of a naphthoquinone compound as a sensitizing agent.
(3) The chemically sensitizable resist has high sensitivity and therefore it can form a resist pattern with high resolution. An example of a resist is described in Japanese Laid-Open Patent Publication No. 60-52845. A resist comprising a compound that produces a cation or an anion when irradiated with light or radiation as a sensitizing agent, and a resin that reacts with the resulting cation or anion moiety to release a silyl group therefrom as a base polymer is known. An example of the main component of this resist is shown in FIG. 17(e), which comprises onium salt as the compound that produces an acid when irradiated with light or radiation, and polytrimethylsiloxystyrene as the polymer which releases a silyl group therefrom.
An example of a resist having the same structure as stated above is described in Proc. Microcircuit Engineering (page 471, 1984, by F. Buiguez, et.al.), which has sensitivity to ultraviolet rays (wavelength: 436 nm) due to the addition of perylene (C.sub.20 H.sub.12) as a sensitizer. FIG. 17(f) further shows a resist described in SPIE (pages 13-20, vol. 920, 1988, by A. Steinmann et.al.), which comprises onium salt as the compound that produces an acid when irradiated with light or irradiation, and polytrimethylsilylphthalaldehyde as a base polymer. Since the onium salt has sensitivity only to far ultraviolet rays (&lt;300 nm), perylene is added to this resist material as a sensitizer so that the resist may have sensitivity to a g-line (436 nm).
However, the above silicon-containing resists have deficiencies as described below.
(1) In the case of a polymerizable one-component resist, an organic solvent is used as a developer in a process of development after exposure. At present, the development by the use of an organic solvent as a developer requires higher cost than that by the use of an alkali aqueous solution as a developer for conventional photoresists. While an alkali developer after dilution with water will not adversely affect human, an organic solvent is insoluble in water, and requires large-scale equipment for decomposition. These entail problems associated with safety in the operation thereof and pollution.
In development by the use of an organic solvent as a developer, the developer penetrates the remaining portions of a resist corresponding to a resist pattern, this makes the resist swollen, thereby lowering the resolution of the resist pattern.
(2) A two-components resist comprising a naphthoquinone compound as a sensitizing agent and a silicon-containing resin which is soluble in an alkali solution as a base polymer can be developed by the use of an alkali aqueous solution. However, since this resist comprises the same naphthoquinone compound as that in conventional photoresists as a sensitizing agent, it has the same level of sensitivity to light as conventional photoresists, which is difficult to be raised.
Since the naphthoquinone compound also has high absorbance to light of short wavelength, the resist is not suitable for lithography by the use of light of short wavelength. The resolution of this kind of conventional resists is not entirely satisfactory, because the dissolving rate of the silicon-containing resin in an alkali solution of a developer has not been optimized.
(3) The chemically sensitizable resists containing silicon has a deficiency in that the resist pattern is also etched with an lower layer during an etching process because of the small content of silicon in the resist. FIG. 9 shows the relationship between the silicon content of the upper resist film and the selection ratio of the etched lower resist film to the etched upper resist film. The data in FIG. 9 is plotted, with the silicon content as abscissa and the selection ratio as the ordinate. When no silicon is contained, the selection ratio with respect to dry etching is zero, indicating that etching of the upper resist film proceeds. With an increase in the silicon content of the upper resist film, the selection ratio increases, thereby improving the resolution of a transferred resist pattern. The silicon content of polytrimethylsiloxystyrene shown in FIG. 17(e) is 14%, and the silicon content of polytrimethylsilylphthalaldehyde shown in FIG. 17(f) is 20%. However, both silicon contents are not sufficient to obtain high resolution, thereby requiring the use of a base polymer having higher silicon content.